1. Field of the Invention
The present invention relates to a sublimation thermal image transfer recording method, and an image receiving sheet for use with the above-mentioned recording method.
2. Discussion of Background
Recently the demand for full color printers is increasing year by year. Representative examples of the recording methods for full color printers now available include the electrophotographic method, ink-jet method, and thermal image transfer recording method. Of these methods, special attention has been paid to the thermal image transfer recording method because of its advantages over the other methods in that the maintenance is easy and the operation is noiseless.
Depending on the kind of employed recording medium, the thermal image transfer recording method can be roughly classified into two types, (i) a thermal fusing image transfer type and (ii) a sublimation thermal image transfer type.
The above-mentioned thermal fusing image transfer recording method (i) is carried out using a thermal image transfer recording medium, namely, an ink sheet, comprising an image transfer layer which comprises a thermofusible material and a coloring agent dispersed therein and an image receiving sheet. The thermal image transfer recording sheet is superimposed on the image receiving sheet in such a manner that the image transfer layer comes into contact with an image-receiving layer of the image receiving sheet, and thermal energy is imagewise applied to the side of the thermal image transfer recording sheet using a laser beam or thermal head, whereby the coloring agent contained in the thermal image transfer recording sheet is fused and transferred to the image receiving sheet.
A sublimation thermal image transfer recording sheet used in the sublimation thermal image transfer recording method (ii) comprises an image transfer layer (namely, a recording layer) which contains a thermally sublimable dye or thermally transferrable dye. When the recording sheet is superimposed on an image receiving layer of the image receiving sheet and thermal energy is imagewise applied to the sublimation thermal image transfer recording sheet, the sublimable dye or transferrable dye can be sublimated or transferred to the image receiving sheet, thereby performing sublimation thermal image transfer recording.
To produce a full-color image, the sublimation thermal image transfer recording method (ii) is generally considered to be advantageous over the thermal fusing image transfer recording method (i) in that color tone can be faithfully reproduced.
In the above-mentioned sublimation thermal image transfer recording method (ii), the sublimation thermal image transfer recording sheet is repeatedly used for image recording in order to curtail the running cost of the image recording. Farther, there has been proposed a so-called n-times-speed mode. In the n-times-speed mode, the transporting speed of the image receiving sheet is made n (n&gt;1) times the transporting speed of the sublimation thermal image transfer recording sheet.
The conventional image receiving sheet for use with the sublimation thermal image transfer recording is still unsatisfactory because preservation stability of the recorded image is poor and durability of the recorded image, such as the light resistance, plasticizer resistance, fingerprint resistance and friction resistance of the recorded image is insufficient. In particular, when the image recorded on the image receiving sheet is allowed to stand at high temperature or exposed to sunlight for a long period of time, the image quality and the image density are easily decreased.
Recently, it has been proposed to prepare a driving license card and a variety of identification cards by the sublimation thermal image transfer recording method. In such a case, the information recorded in the cards must be maintained without deterioration for an extended period of time, so that high durability is required with respect to the recorded image.
There are proposed various methods to solve the above-mentioned problems. For preventing the actions of a the light and oxygen which have an adverse effect on the images recorded on the image receiving sheet, for example, it is proposed that a transparent resin film comprising photo-deterioration inhibitors such as an ultraviolet absorber and an antioxidant is attached to the image-bearing image receiving sheet via an adhesive layer. The images can be shielded from the light and oxygen by such a laminated image receiving sheet, so that the durability of the recorded image is improved to some extent in terms of the light resistance. However, the process for preparing the image-bearing image receiving sheet becomes complicated, and the cost is increased because a laminating machine becomes necessary. In addition, the dye contained in the recorded image tends to transfer to the above-mentioned adhesive layer, thereby causing the image blurring.
There are many proposals for improving the stability of recorded images in terms of light resistance and plasticizer resistance, as disclosed in Japanese Patent Publication 4-55870, and Japanese Laid-Open Patent Applications 4-284291 and 9-66678. According to those applications, after an image comprising a sublimable dye is formed on a single or laminated dye-receiving layer of the image receiving sheet by the application of heat thereto, the image-bearing sheet is heated again using, for example, a thermal head, so as to diffuse the sublimable dye present at the surface portion of the dye-receiving layer into the inside thereof. By this method, good results can be obtained with respect to the preservation stability of the recorded images in terms of the light resistance and plasticizer resistance.
However, even though the image-bearing image receiving sheet is subjected to heat treatment through a sheet member, for example, a sublimable-dye-free area of the recording sheet, the sublimable-dye-free area of the recording sheet is made rough. This is because the thermal head applies the thermal energy of high temperature to the sublimable-dye-free area of the recording sheet as scanning thereon. Further, the glossiness of the image recorded on the image receiving sheet is decreased because the image receiving sheet is scorched by the heat treatment, and the image receiving sheet tends to be curled by the application of heat thereto.
The conventional image receiving sheet for use with the sublimation thermal image transfer recording method comprises a support and a dye-receiving layer formed thereon, comprising a thermoplastic resin with high dyeability with respect to the sublimable dye, such as a polyester resin. Such a conventional image receiving sheet tends to be fused and stick to the thermal image transfer recording sheet in the course of image recording under the application of heat. In particular, when the sublimation thermal image transfer recording is carried out by the n-times-speed mode, the image receiving sheet tends to easily adhere to the recording sheet or to be broken because strong friction is generated between the image receiving sheet and the thermal image transfer recording sheet. Therefore, the image receiving sheet is required to have high heat resistance and lubricity when used in the n-times-speed mode, as compared with the so-called equal-speed mode in which the image receiving sheet and the thermal image transfer recording sheet are transported at equal speed to achieve the image transfer recording.